EUV lithography is anticipated to be the lithographic process of choice for producing future generations of semiconductor devices having linewidths on the order of 32 nm and smaller. The wavelength of the EUV radiation is nominally 13.5 nm, which calls for the use of specialized optics to collect and image the EUV radiation.
One type of EUV optical system used to collect the radiation from the light source is a grazing incidence collector (GIC). Another type is a constant-incidence collector (CIC). A GIC typically comprises one or more concentrically arranged shells configured to receive light from the EUV source at grazing incidence and reflect the light to form a focused illumination beam that first forms an intermediate focus and then creates an illumination region in the far field. The far-field illumination region is preferably uniform to within a specification set by the overall system optical design.
The light sources being considered for EUV lithography include a discharge-produced plasma (DPP) and laser-produced plasma (LPP). The conversion efficiency of these sources is only a few percent, with most of the energy used to generate the EUV radiation converted to infrared, visible and UV radiation and energetic particles that can be incident upon the collector mirror. This radiation causes a substantial thermal load on the GIC mirror. Each GIC mirror shell therefore needs to be cooled so that the heat absorbed by the mirror does not substantially adversely affect GIC mirror performance or damage the GIC mirror.
The same radiation can also heat the spider that holds the GIC shells in a fixed relation to one another. Accordingly, the spider is also preferably cooled so that heat absorbed by the spider is not transferred to the GIC shells, and so that the spider itself does not change its shape. Thus, the GIC mirror is actually part of a GIC mirror system that also includes a GIC shell cooling system and that may also include a spider cooling system.
Essentially all GIC mirror systems for EUV lithography have been used to date only in the laboratory or for experimental “alpha” systems under very controlled conditions. As such, there has been little effort directed to GIC shell cooling systems and spider cooling systems for use in a commercially viable EUV lithography system. In fact, the increasing demand for higher EUV power promises an increased thermal load on the GIC mirror, making such thermal management even more important.
Consequently, more efficient and effective thermal management and cooling systems must be implemented to minimize the potential for optical distortion of the GIC mirror due to the thermal load. The need for thermal management requires a GIC mirror system that is relatively complicated to fabricate. In particular, the GIC mirror needs to be interfaced with the GIC shell cooling system and the optional spider cooling system without causing mechanical distortion of the GIC shells of the GIC mirror.